The present invention relates to an optical fiber coupler or, more particularly, to a reinforced optical fiber coupler provided with a reinforcing base plate adhesively bonded thereto exhibiting an excellent reinforcing effect so that the coupler can be used advantageously for coupling of optical fibers in various kinds of communication systems.
An optical fiber coupler is an indispensable part for the construction of optical communication systems and used for branching and combining of light under transmission through an optical fiber or wave-dividing and wave-synthesizing of light. Among various types of optical fiber couplers, those prepared by working on an optical fiber have excellent coupling adaptability and exhibit remarkably low loss of coupling so that they are widely used in practice for many applications.
An optical fiber coupler of the above mentioned type is prepared by fusing a plural number of optical fibers by heating followed by drawing of the bonded portion under a definite pulling force so as to form a biconical part between the straght-forward columnar parts. A problem in such an optical fiber coupler is the poor mechanical strength due to the decreased cross section of the biconical part so that it is necessary that the optical fiber coupler is used as fixed to a reinforcing member by adhesive bonding. Japanese Patent Kokai 3-107111 proposes a method for fixing an optical fiber coupler to a reinforcing member in which the adhesive used for the adhesive bonding is specified in respect of the Young's modulus and the adhesive bonding is effected at four points. Further, Japanese Patent Kokai 3-78712 proposes to specify the fixing length with an adhesive and spacing between the casing and the optical fiber. According to the disclosures, these methods are effective not only to increase the mechanical strength of the optical fiber coupler but also to improve the coupler so as to pass the temperature characteristic tests such as the heat cycle test by repeating cycles each consisting of heating at +85.degree. C. and chilling at -40.degree. C.
An optical fiber coupler prepared in the above described manner was actually subjected to a heat cycle test to give results that, although no great variation in the characteristics was caused in the ordinary heat cycle test, remarkable changes were caused in the coupling ratio of some of the tested couplers when they were subjected to a very severe heat shock test by the repetition of rapid chilling to -40.degree. C. and rapid heating to +85.degree. C. Accordingly, it is eagerly desired to develop a method for the solution of this problem.